1. Field of the Invention
This invention relates to industrial water cooling towers and particularly to towers of that class which are fire resistant. Novel passive vent means is provided for venting the interior of the tower when a fire occurs to enhance suppression of the combustion process.
2. Description of the Prior Art
Industrial sized water cooling towers have found extensive use in large industrial, business and multiple resident complexes because of their ability to efficiently dissipate large amounts of process or occupancy generated heat to the atmosphere. Cooling towers of this type are found in various areas including factory complexes, chemical processing plants such as petrochemical facilities, near offices, at hospitals, as a part of multi-family apartments or condominiums, as a part of large commercial retail properties, warehouses and electrical generating stations including nuclear power plants.
Cooling towers are constructed of any one of a number of materials including wood, fiberglass, concrete or steel structure, with plastic, wood or ceramic type fill materials. In many instances, uninterrupted operation of the tower is a critical factor because of the importance of continuity of the cooling system to overall process operation, and the particular function of the dependent operating system. The primary job of a cooling tower is generally the efficient cooling of water for processes or facilities. As a result, other design considerations are generally subordinate to the primary cooling function.
However, one usually subordinate design element that can become paramount under certain circumstances relates to fire safety. This is especially true if a tower fire has the potential of endangering personnel in a high occupancy area near the tower, or shut down of the tower because of a fire would adversely affect a process, a production facility or a utility such as a nuclear power plant. Because of these potential hazards, users, insurance carriers, contractors and fire authorities may mandate that the cooling tower be constructed of low combustion materials and of a fire retardant design, or otherwise protected against fire hazards with fire suppression systems such as water sprinklers.
The initial approach to low combustibility tower design was to use non-combustible structure and frame materials combined with a low efficiency, high cost, heavy brick or ceramic type fill, or no fill at all. However, the cost per unit of cooling of these designs was substantial and in most instances not an economically viable option. Later designs retained the non-combustible structures and frame materials previously used, but combined them with lighter weight and less expensive fire retardant fill material having low flame spread characteristics. Low combustibility materials alone however, did not guarantee fire safety.
Configuration and tower design details are critical to actual fire performance of a given model of a tower. Fill materials, although constructed of fire retardant materials, were usually not supported in configurations that substantially limited burning. More fire resistant fill configurations had to be created giving the overall design greater protection from fire damage and shut down. However, concrete and steel support structures were the only acceptable materials for use with the fire retardant fill configurations.
Tower designs which are the most economical and efficient generally have poor fire resistance. This is true of towers having wood frames and plastic fill. Douglas fir for example is an excellent construction material for most industrial cooling tower applications, providing longevity at a very economical price. Towers constructed of fir though are flammable and can completely burn down when exposed to a fire condition and the tower is not in operation. In certain circumstances, inability of the tower to meet fire protection specifications may rule the tower out for a particular application, or require installation of a fire protection sprinkler system which is not only expensive to install but also to maintain under varying ambient conditions. A secondary overall concern with wood towers is the environmental problems associated with leaching of preservative treatment chemicals. Preservatives are necessary to increase the longevity of the wooden cooling tower components under wet conditions.
Fire resistant towers are usually constructed of materials which are not self-propagating from a combustion standpoint. Conventional fire resistant components includes the structural components of the tower such as the upright support members, girts and the like, the fill assembly, the water distribution means overlying the fill assembly, and the casing and fan deck forming an enclosure for the tower.
Each material of construction has certain advantages and disadvantages. Among the factors involved are overall cost, combustibility of the particular material, corrosion resistance, and long term durability effecting longevity of the tower or suitability of the construction materials to a given end use. The ultimate goal would be to provide a cooling tower design which utilizes the highest efficiency components available, with the longest lasting materials of construction, and which cooperate to provide a required cooling function. Protection from material breakdown and strength loss as a result of metal corrosion or wood rot while offering protection from an external risk such as fire has been a long sought but not fully attainable goal. This has been particularly true from a cost benefit standpoint.
Cooling towers which are essentially unprotected from fire hazards, primarily those fabricated from wood, burn rapidly and completely when exposed to risk. Cooling tower fires in some instances may be impossible to extinguish via external fire fighting techniques and therefore require fire detection capable of detecting a combustion event and immediately initiating operation of a fire suppression system.
Fire suppression Systems such as sprinklers have been found to be generally reliable and serve well in several studies of fire incidents involving cooling towers. However, in order to be completely effective, more than one level of sprinklers are usually required on very tall tower configurations. However, sprinkler systems are very maintenance dependent insofar as corrosion is concerned and offer particularly difficult design considerations insofar assurance of water supply under freezing conditions.
Not only must the system provide rapid fire detection with low probability of false signaling, necessitating complex and costly detectors such as thermistors or the like. In addition, the water release mechanism must be positive and instantaneously responsive to fire detection. Sprinkler heads having melt out plugs have been employed, but squib actuated deluge valves are preferred because of their faster reaction time. Although these systems have proven to be effective as installed, they are expensive and extremely difficult to maintain on a regular basis over intervals of time that normally involves many years where there is no functional need for the system to operate.
Because sprinkler systems are expensive to install and require many specific care actions and programed maintenance, cooling tower users have sought to eliminate the need for such systems based on tower designs. These alternative design concepts have for the most part relied upon the use of non-combustible materials which are much more expensive than wood. Furthermore, the designs must meet customer acceptance standards and desirably comply with industry standards such as those receiving Factory Mutual (FM) approval, without the use of sprinkler protection. This entails not only structural protection in fires, but also limited combustion spread, especially laterally in the fill assembly, which represents the area of highest internal BTU content.
FM approval has typically been based on testing of tower mock ups for full tower sections for fire resistance. The FM approval process is based on placement of a very substantial ignitor in the lower part of the tower, and then observation of the effect of that ignition on the test cell or tower as a whole. A one foot by one foot plan area pan containing heptane to a depth of three inches is placed in the tower below the fill assembly and ignited. FM has not published specific judgment criteria, but instead issues approvals case by case based on test observations.
With the advent of synthetic resin framing components for cooling towers, as for example polyester resin reinforced with fiberglass, usually referred to as FRP or GRP, the provision of a fire resistant cooling tower which has required strength, durability and longevity characteristics, has become closer to reality. The use of FRP in the construction of cooling towers has slowly evolved over the years. Glass fiber reinforced synthetic resin components such as fan stacks, fan blades, fill support grids and wood tower braced diagonal connectors have been used for a number of years and have established FRP as a durable material in the corrosive cooling tower environment. In more recent years, virtually the entire components of a cooling tower have utilized FRP materials to provide effective corrosion resistance while retaining required structural strength. Exemplary is the "Four-way Crossflow Water Cooling Tower" of U.S. Pat. No. 4,788,013. Towers of the type disclosed in the '013 patent using alternative materials have become closer in cost to prior wood designs, particularly when the properties of cooling efficiency, superior corrosion resistance, long term longevity and overall maintenance and replacement costs are taken into account.
FRP cooling tower design interests have now focused on producing a low combustibility, low fire risk design in pultruded fiberglass structural material using high efficiency low flame spread fill materials such as polyvinyl chloride (PVC).
The assignee hereof has obtained a number of FM approvals on crossflow and counterflow cooling towers. These designs have been characterized by steel or concrete framing and PVC fill and eliminators in various configurations. Also approved have been fiberglass reinforced polyester fan blades, fan cylinders and distribution pipes along with PVC distribution piping and polypropylene type adaptors and nozzles. Approved tower sizes have ranged from relatively small towers, 4 feet by 4 feet by 6 feet to very large towers having a diameter as much as 400 hundred feet and a concrete shell 500 feet tall.
An FM approved tower incorporating a non-combustible ceramic tile fill is very inefficient in cooling capacity, is size limited based on fill weight, and is a very expensive design. A more recently approved tower design employs an extra cell for redundancy and an impenetrable fire barrier between each cell. The extra cell is required because a whole tower segment between any barrier location is subject to total fire exposure. Manifestly, provision of an extra cell protected by a fire barrier is a very expensive and therefore undesirable attempt to solve the fire hazard problem. PVC fill in a combustible support frame requires a substantial fire barrier and significant extra tower capacity. Burning cannot be controlled by design in any current FRP framed, PVC filled tower design.
A number of fire hazards exist in connection with cooling tower installations. The primary fire risks are associated with: 1) electrical equipment malfunctions and shorts, principally occurring in fan motors or junction boxes; 2) lightning strikes; 3) welding/cutting torch sparks from on or near the cooling tower; 4) sparks from an external source in the area such as an incinerator; and 5) careless storage of combustibles on, near or under the cooling tower, creating ignition sensitivity problems. Contrary to what would be expected, studies have shown that at least a third of cooling tower fires occur while the tower is in operation. The principle fire risk areas are the fan deck which is exposed to external sparks, and the fill assembly, because of its combustible nature and large BTU content in a limited internal area.
As a consequence, principle efforts to limit fire risks have heretofore for the most part been directed toward use of non-combustible materials, especially the fill components, and structural members, by configuration alternatives to limit fill combustion, by adding well maintained sprinkler systems with adequate water supply that is not subject to freeze up or corrosion, by adding lightning protection, by careful siting to avoid high risk locations, by specific management control of cutting and welding activities because of the high number of fires which result from these sorts of accidents, and by initiating emergency reaction readiness planning.